Xeroradiographic material and method of making same

ABSTRACT

A xeroradiographic material comprising a substrate at least one surface of which is electrically conductive, and an X-ray sensitive layer provided on the conductive surface of the substrate and essentially consisting of an organic binder, γ-form crystal grains of a bismuth oxide-based compound oxide and n-type semiconductor grains dispersed in the organic binder. The material is made by dispersing these grains in an organic binder solution, applying the dispersion onto the conductive surface of the substrate, drying the coat of the dispersion at a temperature in a range not lower than the boiling point of the solvent but below the softening point of the organic binder to form an X-ray sensitive layer, and heat-treating the X-ray sensitive layer at a temperature in a range not lower than the softening point of the organic binder but below the temperature at which the organic binder begins decomposing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a xeroradiographic material and a method ofmaking same.

2. Description of the Prior Art

Recently, body-centered cubic (γ-form) crystals of bismuth oxide-basedcompound oxides represented by the following general formula:

    Bi.sub.x MO.sub.n

in which M designates at least one of germanium, silicon, titanium,gallium and aluminum, x denotes a number satisfying the condition10≦x≦14, and n denotes a number of oxygen atoms stoichiometricallydetermined depending on M and X, have attracted attention for use asphotoconductive substances. The γ-form crystalline compound oxidesbecome electrically conductive when exposed to X-rays. Therefore, it hasbeen proposed in Japanese Unexamined Patent Publication No.53(1978)-43531 to use them as photoconductive substances inxeroradiographic materials for forming electrostatic latent images byexposure to X-rays.

Xeroradiographic materials comprising a conductive substrate, and anX-ray sensitive layer formed of a dispersion containing the γ-formcrystal grains of the above-mentioned compound oxides in an organicbinder and provided on the conductive substrate exhibit a remarkablyhigh sensitivity to X-rays. For example, it is known to use axeroradiographic material comprising a conductive substrate, and anamorphous selenium layer deposited on the conductive substrate formammography and the like. In general, compared with the xeroradiographicmaterial comprising an amorphous selenium layer, the xeroradiographicmaterials provided with an X-ray sensitive layer formed of a dispersioncontaining the γ-form crystal grains of the above-mentioned compoundoxides in an organic binder exhibit a sensitivity to X-rays about fiveto ten times higher.

When xeroradiographic materials are used for xeroradiography for themedical diagnostic purposes, it is desirable that the materials be assensitive as possible to X-rays in order to minimize the exposure dosewhich patients receive. For this reason, it is desired to furtherincrease the sensitivity of the xeroradiographic material comprising anX-ray sensitive layer formed of a dispersion containing the γ-formcrystal grains of the above-mentioned compound oxides in an organicbinder.

SUMMARY OF THE INVENTION

The primary object of the present invention is to provide axeroradiographic material comprising an X-ray sensitive layer formed ofa dispersion containing the γ-form crystal grains of bismuth oxide-basedcompound oxides in an organic binder, which exhibits an improvedsensitivity to X-rays.

Another object of the present invention is to provide a method of makinga xeroradiographic material comprising an X-ray sensitive layer formedof a dispersion containing the γ-form crystal grains of bismuthoxide-based compound oxides in an organic binder, which exhibits animproved sensitivity to X-rays.

The specific object of the present invention is to provide axeroradiographic material comprising an X-ray sensitive layer formed ofa dispersion containing the γ-form crystal grains of bismuth oxide-basedcompound oxides in an organic binder, which exhibits an improvedsensitivity to X-rays and can be made in a simple way, and a method ofmaking same.

To accomplish the above objects, the inventors conducted various studieson X-ray sensitive layers containing the γ-form crystal grains of thebismuth oxide-based compound oxides dispersed in an organic binder, andfound that sensitivity of the X-ray sensitive layer to X-rays can beimproved if n-type semiconductor grains are dispersed in an organicbinder together with the γ-form crystal grains of the compound oxidesdescribed above to form an X-ray sensitive layer.

The xeroradiographic material in accordance with the present inventioncomprises a substrate at least one surface of which is electricallyconductive, and an X-ray sensitive layer provided on the electricallyconductive surface of said substrate, said X-ray sensitive layeressentially consisting of an organic binder, γ-form crystal grains ofthe bismuth oxide-based compound oxides and n-type semiconductor grainsdispersed in said organic binder.

The xeroradiographic material in accordance with the present inventioncan be made by dispersing γ-form crystal grains of the bismuthoxide-based compound oxides and n-type semiconductor grains in anorganic binder solution containing an organic binder dissolved in asolvent, applying the dispersion obtained onto an electricallyconductive surface of a substrate at least one surface of which iselectrically conductive, heating and drying the coat of the dispersionat a temperature within a range not lower than the boiling point of saidsolvent but below the softening point of said organic binder to form anX-ray sensitive layer on the electrically conductive surface. It wasfound that, if the X-ray sensitive layer thus formed on the electricallyconductive surface of the substrate is further heat-treated at atemperature within a range not lower than the softening point of theorganic binder but below the temperature at which the organic binderstarts decomposing, the resulting X-ray sensitive layer exhibits afurther improved sensitivity to X-rays.

Accordingly, the method of making a xeroradiographic material inaccordance with the present invention comprises the steps of:

(a) dispersing γ-form crystal grains of the bismuth oxide-based compoundoxide defined above and n-type semiconductor grains in an organic bindersolution containing an organic binder dissolved in a solvent,

(b) applying the dispersion thus obtained onto an electricallyconductive surface of a substrate at least one surface of which iselectrically conductive,

(c) heating and drying the coat of the dispersion at a temperaturewithin a range not lower than the boiling point of said solvent butbelow the softening point of said organic binder to form on saidelectrically conductive surface an X-ray sensitive layer essentiallyconsisting of said organic binder, and said γ-form crystal grains of thebismuth oxide-based compound oxide and said n-type semiconductor grainsdispersed in said organic binder, and thereafter

(d) heat-treating the X-ray sensitive layer at a temperature within arange not lower than the softening point of said organic binder butbelow the temperature at which said organic binder begins decomposing.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will hereinbelow be described in further detail.

The xeroradiographic material in accordance with the present inventionis made by the process as described below.

First, a dispersion is prepared by dispersing the γ-form crystal grainsof the bismuth oxide-based compound oxide defined above and the n-typesemiconductor grains in an organic binder solution containing a suitableorganic binder dissolved in a suitable solvent. The organic bindersshould be organic substances which do not adversely affect the γ-formcrystal grains of the compound oxides and the n-type semiconductorgrains and which can form a layer of a dispersion containing thesegrains. In general, any organic substances that are known to be capableof being used as binders in light-sensitive layers ofelectrophotographic materials can be used as the organic binders in thepresent invention. The solvents for dissolving the organic binders areselected suitably according to the kinds of the organic binders. In thepresent invention, from the view point of the sensitivity of thexeroradiographic materials obtained, it is also possible to use organicbinders of the type exhibiting the charge carrier conveying capabilityand used in xeroradiographic materials disclosed, for example, inJapanese Unexamined Patent Publication No. 56(1981)-5549. Thispublication describes xeroradiographic materials comprising an X-raysensitive layer formed by dispersing γ-form crystal grains of thebismuth oxide-based compound oxide in organic binders exhibiting thecharge carrier conveying capability. Examples of the organic bindersexhibiting the charge carrier conveying capability and the solventssuitable for such organic binders are described in the publicationmentioned above.

The γ-form crystal grains of the bismuth oxide-based compound oxides canbe obtained by pulverizing γ-form crystals (single crystal orpolycrystal) of the compound oxides prepared by a known method such asCzochralski's method. The γ-form crystal grains of the compound oxidesgenerally have a grain size of 1,000 μm or less, preferably 200 μm orless, more preferably 100 μm or less. In the present invention, from theview point of the sensitivity of the xeroradiographic materialsobtained, the bismuth oxide-based compound oxides represented by thegeneral formula mentioned above in which M denotes germanium and/orsilicon, i.e. those represented by the formula:

    Bi.sub.x (Ge.sub.1-y, Si.sub.y)O.sub.n

in which x and n have the meanings as defined above, and y denotes anumber satisfying the condition O≦y≦1, are particularly preferable.Among these particularly preferable compound oxides, those representedby the formula just mentioned above in which x denotes 12, y denotes 0,and n denotes 20 (i.e. Bi₁₂ GeO₂₀), and those of the formula in which xdesignates 12, y designates 1, and n designates 20 (i.e. Bi₁₂ SiO₂₀) arefurther preferable.

In the present invention, the n-type semiconductor grains may be grainsof C (diamond), Mg₂ Al₂ O₄, β-SiC, TiO₂, V₂ O₅, V₂ O₄, MnO₂, Fe₂ O₃,FeS₂, ZnO, Cu₂ S, CuInSe₂, ZnS, ZnSe, ZnTe, GeSe, SrO, Nb₂ O₅, Nb₂ O₄,Nb₂ O₃, MoO₃, MoS, MoS₂, β-Ag₂ S, β-Ag₂ Se, β-Ag₂ Te, CdS, InSe, SnO,SnO₂, SnSe, Sb₂ O₄, Ta₂ O₃, Ta₂ O₅, WO₃, HgSe, Bi₂ S₃, Bi₂ Se₃, CeO₂,Nd₂ O₃, PbCrO₄, HgS and the like. Of these n-type semiconductor grains,those of ZnO, CdS, WO₃ and TiO₂ are particularly preferable because ofhigher sensitivity of the xeroradiographic materials obtained. Ingeneral, n-type semiconductor grains having a grain size equial to orsmaller than that of the γ-form crystal grains of the compound oxidesare employed in the present invention.

The n-type semiconductor grains are generally used in a ratio within therange between 0.1% and 50% by volume, preferably between 1% and 30% byvolume, more preferably between 3% and 10% by volume, based on the totalof the γ-form crystal grains of the compound oxides and the n-typesemiconductor grains. The organic binders are generally used in a ratiowithin the range between 1% and 90% by volume, preferably between 10%and 70% by volume, more preferably between 20% and 50% by volume, basedon the total of the γ-form crystal grains of the compound oxides, then-type semiconductor grains and the organic binders.

The dispersion is prepared by adding the γ-form crystal grains of thecompound oxides and the n-type semiconductor grains to an organic bindersolution containing an organic binder dissolved in a solvent, andintimately stirring the resulting mixture by an appropriate means. Inthis case, it is preferable that the γ-form crystal grains of thecompound oxides and the n-type semiconductor grains be dispersed asprimary particles in the organic binder solution.

Thereafter, the dispersion prepared as described above is uniformlyapplied to an electrically conductive surface of a substrate at leastone surface of which is electrically conductive. For this purpose, anysubstrate may be used insofar as the surface on which the dispersion isapplied, i.e. the surface on which an X-ray sensitive layer is formed,is electrically conductive. In general, metal plates are used as thesubstrates. Of the metal plates, aluminium plates, stainless steelplates or zinc plates are preferably used for there economy and ease ofhandling. The dispersion may be applied to the substrate by an ordinarymethod using a wire bar, a doctor blade, a roll coater, a knife coateror the like. In general, the dispersion is applied to the substrate insuch an amount that the thickness of the X-ray sensitive layer afterheating and drying is in the range between 10 μm and 2,000 μm,preferably between 30 μm and 800 μm, more preferably between 100 μm and400 μm.

The coat of the dispersion applied to the substrate is then heated anddried. The heating and drying process is conducted for a sufficientlength of time at a temperature within a range not lower than theboiling point of the solvent contained in the dispersion but below thesoftening point of the organic binder contained in the dispersion. Inthis process, the solvent is removed and an X-ray sensitive layer isformed on the substrate. The X-ray sensitive layer thus formedessentially consists of the organic binder, and the γ-form crystalgrains of the compound oxide and the n-type semiconductor grainsdispersed in the organic binder.

The xeroradiographic material in accordance with the present inventionobtained by the method described above exhibits a higher sensitivitythan xeroradiographic materials made in the same way as described above,except that the n-type semiconductor grains are not used. The extent ofthe improvement in sensitivity achieved by the use of the n-typesemiconductor grains differs according to the kind of the n-typesemiconductor grains used, and the like. However, for example, when ZnO,CdS, WO₃ or TiO₂ grains are used as the n-type semiconductor grains, thesensitivity of the xeroradiographic material obtained therefrom is, ingeneral, about 1.5 to 2 times that of xeroradiographic material madewithout using the n-type semiconductor grains. The reason why the X-raysensitive layer formed by dispersing the n-type semiconductor grainstogether with the γ-form crystal grains of the compound oxides in anorganic binder exhibits an improved sensitivity has not completelyclarified. However, it is presumed that the improved sensitivity isobtained for the reason described below. Namely, in the case of theX-ray sensitive layer formed of a dispersion containing only the γ-formcrystal grains of the compound oxide in an organic binder, chargecarriers generated in a γ-form crystal grain of the compound oxide dueto excitation by X-rays can move only in that grain because the organicbinder is an electrically insulating substance. On the other hand, inthe case of the X-ray sensitive layer formed of a dispersion containingthe n-type semiconductor grains together with the γ-form crystal grainsof the compound oxide in an organic binder, it is presumed that thecharge carriers, particularly electrons, generated in a γ-form crystalgrain of the compound oxide can transfer more easily from the γ-formcrystal grain into the n-type semiconductor grains.

When the X-ray sensitive layer formed on the substrate by the methoddescribed above is further heat-treated at a temperature within a rangenot lower than the softening point of the organic binder contained inthe X-ray sensitive layer but below the temperature at which the organicbinder begins decomposing, the sensitivity of the X-ray sensitive layer(i.e. the sensitivity of the xeroradiographic material) is furtherimproved. In this heat treatment step, the heating temperature and theheating time must be strictly controlled to achieve sufficientimprovement in sensitivity. Namely, when heat treatment is conducted ata temperature equal to or near to the softening point of the organicbinder, it is necessary to conduct heat treatment for a relatively longtime in order to obtain sufficient improvement in sensitivity. When heattreatment is conducted in the vicinity of the temperature at which theorganic binder begins decomposing, a sufficient improvement insensitivity can be obtained in a relatively short heating time.Particularly, when heat treatment is conducted in the vicinity of thetemperature at which the organic binder begins decomposing, it isnecessary to very strictly control the heat treatment time so that theX-ray sensitive layer may not be burnt or adversely affected. Ingeneral, the X-ray sensitive layer heat-treated as described aboveexhibits a sensitivity about 2 to 3 times higher than that before heattreatment. It is presumed that the sensitivity of the X-ray sensitivelayer is improved by the heat treatment described above because theorganic binder is softened by heat and, therefore, the contactingcondition between the γ-form crystal grains of the compound oxide andthe n-type semiconductor grains is improved.

The present invention will further be illustrated by the followingnonlimitative examples.

EXAMPLE 1

Grains having a grain size in the range between 63 μm and 105 μm wereobtained by pulverizing Bi₁₂ GeO₂₀ γ-form single crystals prepared bythe Czochralski method, and classifying the pulverized grains.

On the other hand, a polyester resin ("Vlyon 200" available from ToyoboCo., Ltd., Japan) was dissolved in tetrahydrofuran to prepare a 15 wt. %strength polyester resin solution.

Thereafter, the Bi₁₂ GeO₂₀ γ-form crystal grains obtained as describedabove and n-type semiconductor grains were added to the polyester resinsolution prepared as described above. The mixture thus obtained wasintimately stirred to prepare a dispersion. In this case, the mixingratio of the Bi₁₂ GeO₂₀ γ-form crystal grains, the n-type semiconductorgrains and the polyester resin was set so that their volume ratio afterdrying was 7:1:2. In this way, four kinds of dispersions were preparedby using ZnO grains ("SAZEX 4000" available from Sakai Chemical IndustryCo., Ltd., Japan), CdS grains (available from Yamanaka Kagaku K.K.,Japan), WO₃ grains (available from Mitsuwa Kagaku K.K., Japan), and TiO₂grains (available from Yamanaka Kagaku K.K., Japan) as the n-typesemiconductor grains.

Each of the dispersions prepared as described above was uniformlyapplied onto a 0.3 mm-thick aluminium plate by using a wire bar so thatthe thickness of the coat after drying was about 400 μm. Then, the coatof each dispersion applied on the aluminium plate was heated and driedfor two hours at a temperature of 60° C. in the atmosphere, and furtherfor 18 hours at a temperature of 140° C. in the atmosphere to form anX-ray sensitive layer on the aluminium plate. In this way,xeroradiographic material specimens Nos. 1 to 4 were obtained.

On the other hand, comparative xeroradiographic material specimens Nos.5 and 6 were prepared in the same way as described above, except thatthe n-type semiconductor grains were not used and that the mixing ratioof the Bi₁₂ GeO₂₀ γ-form crystal grains to the polyester resin was setso that the volume ratio between them after drying was 7:3 and 4:1,respectively.

Each of the specimens obtained as described above was thencorona-discharged at a corona voltage of -5 kV in the dark, and exposedto X-rays while the surface potential was measured by use of a surfacepotential meter (SSV-II-50 available from Kawaguchi Denki K.K., Japan).The conditions for exposure to X-rays were as follows:

X-ray generator/controller: KXO-15 available from Toshiba Corporation,Japan.

X-ray tube: DRX-190A available from Toshiba Corporation, Japan.

Tube voltage: 80 kV.

Tube current: 1 mA.

Distance from focal point to xeroradiographic material: 1 m.

Exposure mode: Continuous.

Exposure dose rate at xeroradiographic material surface: 4.7 mR/sec.(measured with Model 500 dosimeter available from VICTOREEN Company).

To evaluate the sensitivity to X-rays, the exposure dose (halfattenuation exposure dose) required to reduce the surface potential ofthe xeroradiographic material just prior to the exposure to X-rays(initial potential) to half was measured. The results were as shown inTable 1 below.

As clearly shown in Table 1, the xeroradiographic materials (specimensNos. 1 to 4) in accordance with the present invention exhibited asensitivity about 1.5 to 2 times higher than those of thexeroradiographic materials (specimens Nos. 5 and 6) which were made inthe same way as in the present invention, except that the n-typesemiconductor grains were not used.

                  TABLE 1                                                         ______________________________________                                                n-Type                                                                Specimen                                                                              semiconductor                                                                             Initial    Half attenuation                               No.     grains      Potential (V)                                                                            exposure dose (mR)                             ______________________________________                                        1       ZnO         -500       28                                             2       CdS         -590       32                                             3       WO.sub.3    -500       33                                             4       TiO.sub.2   -540       35                                             5       --          -520       54                                             6       --          -500       56                                             ______________________________________                                    

EXAMPLE 2

A resin mixture containing the same polyester resin as used in Example 1and an alkyd resin ("Beckolite" available from Dainippon Ink AndChemicals, Incorporated, Japan) in a volume ratio 17:3 was dissolved ina solvent mixture containing methyl ethyl ketone and toluene in a weightratio 1:4. In this way, a 20 wt. % strenght solution of the resinmixture was prepared.

Thereafter, the same Bi₁₂ GeO₂₀ γ-form crystal grains as used in Example1 and ZnO grains were added to the solution of the resin mixture, andthe mixture was intimately stirred to prepare a dispersion. In thiscase, the mixing ratio of Bi₁₂ GeO₂₀ γ-form crystal grains, the ZnOgrains and the resin mixture was set so that their volume ratio afterdrying was 15:1:4.

The dispersion thus obtained was then uniformly applied onto a 0.3mm-thick aluminium plate by using a wire bar in the same way asdescribed in Example 1 so that the thickness of the coat after dryingwas about 300 μm. Then, the coat of the dispersion applied on thealuminium plate was heated and dried for two hours at a temperature of60° C. in the atmosphere, and further for 26 hours at a temperature of140° C. in a vacuum dryer to form an X-ray sensitive layer on thealuminium plate. In this way, three xeroradiographic material specimensof the same type were made, one of which was taken as specimen No. 7.

Two remaining specimens were heat-treated for one hour at a temperatureof 220° C. (which was higher than the softening point of theabove-mentioned resin mixture but lower than the temperature at whichthe resin mixture begins decomposing) in the atmosphere (first heattreatment). One of the two xeroradiographic material specimens thusheat-treated was taken as specimen No. 8. Thereafter, the other of thetwo specimens was further heat-treated for three minutes at atemperature of 330° C. (which was higher than the softening point of theabove-mentioned resin mixture but lower than the temperature at whichthe resin mixture begins decomposing) in nitrogen gas (second heattreatment). The xeroradiographic material thus heat-treated was taken asspecimen No. 9.

The half attenuation exposure dose was measured on specimen Nos. 7 to 9thus obtained in the same way as described in Example 1. The resultswere as shown in Table 2.

As clearly shown in Table 2, the xeroradiographic material (specimen No.8) comprising a X-ray sensitive layer formed thereon and subjected tothe first heat treatment, and the xeroradiographic material (specimenNo. 9) subjected to the first heat treatment and the second heattreatment exhibited a sensitivity about 2 to 2.5 times higher than thoseof the xeroradiographic material (specimen No. 7) which was notsubjected to the heat treatment.

                  TABLE 2                                                         ______________________________________                                        Specimen    Initial      Half attenuation                                     No.         Potential (V)                                                                              exposure dose (mR)                                   ______________________________________                                        7           -500         21                                                   8           -500         11                                                   9           -500          8                                                   ______________________________________                                    

What is claimed is:
 1. A xeroradiographic material comprising asubstrate at least one surface of which is electrically conductive, andan X-ray sensitive layer provided on the electrically conductive surfaceof said substrate, said X-ray sensitive layer essentially consisting ofan organic binder, (i) γ-form crystal grains of a bismuth oxide-basedcompound oxide represented by the following general formula:

    Bi.sub.x MO.sub.n

in which M designates at least one of germanium, silicon, titanium,gallium and aluminum, x denotes a number satisfying the condition10≦x≦14, and n denotes a number of oxygen atoms stoichiometricallydetermined depending on M and x, and (ii) inorganic n-type semiconductorgrains, said constituents (i) and (ii) being dispersed in said organicbinder.
 2. A xeroradiographic material as defined in claim 1 whereinsaid n-type semiconductor grains are selected from the group consistingof ZnO, CdS, WO₃ and TiO₂ and mixtures thereof.
 3. A xeroradiographicmaterial as defined in claim 1 or 2 wherein said organic binder exhibitscharge carrier conveying capability.
 4. A xeroradiographic material asdefined in any of claims 1 or 2 wherein M in said general formuladesignates germanium and/or silicon.
 5. A xeroradiographic material asdefined in claim 1 wherein said bismuth oxide-based compound oxide isrepresented by said general formula in which M designates germanium, xdenotes 12, and n denotes
 20. 6. A xeroradiographic material as definedin claim 1 wherein said bismuth oxide-based compound oxide isrepresented by said general formula in which M designates silicon, xdenotes 12, and n denotes
 20. 7. A method of making a xeroradiographicmaterial comprising the steps of:(a) dispersing (i) γ-form crystalgrains of a bismuth oxide-based compound oxide represented by thefollowing general formula:

    Bi.sub.x MO.sub.n

in which M designates at least one of germanium, silicon, titanium,gallium and aluminum, x denotes a number satisfying the condition10≦x≦14, and n denotes a number of oxygen atoms stoichiometricallydetermined depending on M and x, and (ii) n-type semiconductor grains inan organic binder solution containing an organic binder dissolved in asolvent, (b) applying the dispersion thus obtained onto an electricallyconductive surface of a substrate at least one surface of which iselectrically conductive, and (c) heating and drying the coat of thedispersion at a temperature within the range not lower than the boilingpoint of said solvent but below the softening point of said organicbinder to form on said electrically conductive surface an X-raysensitive layer essentially consisting of said organic binder, and saidγ-form crystal grains of the bismuth oxide-based compound oxide and saidn-type semiconductor grains dispersed in said organic binder.
 8. Amethod as defined in claim 7 wherein said n-type semiconductor grainsare selected from the group consisting of ZnO, CdS, WO₃ and TiO₂ andmixtures thereof.
 9. A method as defined in claim 7 or 8 wherein saidorganic binder exhibits charge carrier conveying capability.
 10. Amethod as defined in any of claims 7 or 8 wherein M in said generalformula designates germanium and/or silicon.
 11. A method as defined inclaim 7 wherein said bismuth oxide-based compound oxide is representedby said general formula in which M designates germanium, x denotes 12,and n denotes
 20. 12. A method as defined in claim 7 wherein saidbismuth oxide-based compound oxide is represented by said generalformula in which M designates silicon, x denotes 12, and n denotes 20.13. A method as in claim 7 including the step, after step (c), ofheat-treating the X-ray sensitive layer at a temperature within therange no lower than the softening point of said organic binder but belowthe temperature at which said organic binder begins decomposing.